MIMO (Multiple-Input Multiple-Output) transmission techniques that allow, with the use of a plurality of antennas for transmission and reception, simultaneous communication of a plurality of different data sequences (data streams) in the same frequency band by spatial multiplexing have been practically used in wireless LANs, cellular systems, and the like. In single user MIMO that allows transmission of a plurality of different data sequences to one certain wireless reception device (terminal device) by spatial multiplexing, there has been a method for precoding a transmit signal at a wireless transmission device (base station device, access point) before transmission in order to improve the performance of the wireless reception device for separating and detecting a plurality of data sequences. Furthermore, in next-generation cellular systems, wireless LANs, and the like, there has been proposed a system in which a wireless transmission device includes transmit antennas the number of which is significantly larger than the number of receive antennas included in a wireless reception device. In order to make effective use of the transmit antennas at the wireless transmission device in this system to further improve system throughput, there has been proposed multi-user MIMO that provides MIMO multiplexing of data sequences addressed to a plurality of wireless reception devices (users).
However, it is difficult for each of a plurality of wireless reception devices that receive signals multiplexed in multi-user MIMO to know the signals received by the other wireless reception devices. Under these circumstances, therefore, the performance will significantly degrade due to Multi-User Interference (MUI) caused between streams addressed to individual users. Accordingly, there have been proposed several methods that enable a wireless transmission device to generate a transmit signal capable of MUI suppression at the time of reception at wireless reception devices without imposing large loads on the wireless reception devices if the wireless transmission device knows CSI (Channel State Information), which is channel state information from each transmit antenna at the wireless transmission device to each receive antenna at each wireless reception device (NPL 1).
For example, there is a method for precoding a transmit signal at a wireless transmission device before transmission so as to enable a wireless reception device to receive the signal with MUI suppressed at the time of reception. Examples of this method include linear precoding (LP) for precoding a transmit signal through linear processing, such as Zero-forcing (ZF) precoding for weighting a transmit signal (multiplying a transmit signal by W=H−1) using as a weight matrix (linear filter) W the inverse matrix H−1 (or pseudo-inverse matrix H†=HH(HHH)−: the superscript H denotes the Hermitian conjugate) of a channel matrix H whose elements correspond to complex channel gain between each transmit antenna and each receive antenna at each wireless reception device, which is determined from CSI, and Minimum Mean Square Error (MMSE) precoding for weighting a transmit signal with a weight matrix (linear filter) W=HH(HHH+αI)−1 (I denotes a unit matrix and α denotes a normalization factor) determined by MMSE criteria.
Other examples include non-linear precoding (NLP) for precoding a transmit signal through non-linear processing to subtract an interference signal component determined from CSI from the transmit signal in advance and to encode the interference-subtracted signal to constellation points where transmission power is reduced in a signal space in order to suppress an increase in transmission power after interference subtraction. One type of non-linear precoding, Tomlinson-Harashima Precoding (THP), which allows suppression of an increase in transmission power by performing a modulo (remainder) operation on signals at both a wireless transmission device and wireless reception devices, has been proposed (NPL 2, NPL 3, NPL 4). In this case, wireless reception devices on the receive side also perform a modulo operation on received signals in a manner similar to that for transmission. Here, a modulo operation Modτ with a modulo width τ is represented by Expression (1) if a complex vector representing an interference-subtracted signal on the transmit side or a received signal on the receive side is represented by ν. In Expression (1), j denotes the imaginary unit, floor(a) denotes a maximum integer not exceeding a, and Re(ν) and Im(ν) denote the real part (corresponding to the in-phase component of a signal) and the imaginary part (corresponding to the quadrature component of a signal) of the complex number ν, respectively. The term “non-linear processing”, as used here, mainly refers to processing that uses processing in which discontinuity points are present in the output of the modulo operation or the like.
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                                            Mod            τ                    ⁡                      (            v            )                          =                  v          -                                    floor              (                                                                    Re                    ⁡                                          (                      v                      )                                                        +                                      τ                    2                                                  τ                            )                        ·            τ                    -                      j            ·                          floor              (                                                                    Im                    ⁡                                          (                      v                      )                                                        +                                      τ                    2                                                  τ                            )                        ·            τ                                              (        1        )            
Another scheme has been proposed that takes into account performance degradation, called Modulo-Loss at a low Signal to Noise power Ratio (SNR), which is caused by modulo operation in THP, in which processing similar to linear precoding is performed without application of a modulo operation to a terminal with a low MUI and a small effect of transmission power suppression achieved by the modulo operation, whereas non-linear precoding based on THP is performed with application of a modulo operation to only a terminal with a high MUI and a large effect of transmission power suppression achieved by the modulo operation (NPL 6).
On the other hand, a method for identifying a plurality of wireless reception devices to which data sequences spatially multiplexed into multi-user MIMO signals, which are transmitted from a wireless transmission device, are addressed, namely, a method for determining combinations (groups) of wireless reception devices for which spatial multiplexing with multi-user MIMO processing is performed and embedding group IDs (GIDS) each identifying one of the groups into control information of transmit frames when the frames are sent via multi-user MIMO transmission, has been proposed in wireless LAN standard IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.11ac under establishment (NPL 7). Here, each wireless reception device may belong to a plurality of groups. A wireless transmission device (access point) determines combinations (groups) of wireless reception devices for which spatial multiplexing with multi-user MIMO is performed, before multi-user MIMO transmission, and adds group IDs each identifying one of the groups to notify each of the wireless reception devices of a group to which the wireless reception device belongs. The wireless transmission device also notifies each of the wireless reception devices of the group configurations.